One cannot hear sound on the moon because it has no:

Answer: Atmosphere.

Sound is a mechanical wave: it propagates only through a material medium (a gas, liquid, or solid) by making the particles of that medium vibrate. It cannot travel through a vacuum, because there are no particles to carry the vibration.

The Moon has essentially no atmosphere — its surface is a near-perfect vacuum (its thin exosphere is about a trillion times less dense than Earth's air at sea level). With no air to act as a medium, sound waves cannot propagate, so a person standing on the Moon hears nothing from outside their suit.

This is why astronauts on the Moon communicate by radio: radio waves are electromagnetic and travel through a vacuum, whereas the sound of their voices is carried only by the air sealed inside their helmets.